Why do supporters defend Obama on non-issues, and call McCain a liar on non-issues?

Because his supporters "support" him so when a republican brings up a non-issue then you can bet his supporters will probably try to say something advocating his position. They probably call McCain a liar because something he said is not true. If it's about a non-issue, then they would say something like, "Hey, you lied about that non-issue, you liar."